Handle missing arg pointer in get_call_error_text

I had a situation coming from godot-python where the caller of
Variant::get_call_error_text() passed null for `p_argptrs`. In
addition to fixing that in the caller, seems like good practice to
defend against that situation in the callee to prevent a crash.

So this patch just substitutes some semi-useful text for the source
type name and keeps going so the user's actual error gets emitted.

core/variant.cpp

@@ -3167,7 +3167,11 @@ String Variant::get_call_error_text(Object *p_base, const StringName &p_method,
 
 	if (ce.error == Variant::CallError::CALL_ERROR_INVALID_ARGUMENT) {
 		int errorarg = ce.argument;
-		err_text = "Cannot convert argument " + itos(errorarg + 1) + " from " + Variant::get_type_name(p_argptrs[errorarg]->get_type()) + " to " + Variant::get_type_name(ce.expected) + ".";
+		if (p_argptrs) {
+			err_text = "Cannot convert argument " + itos(errorarg + 1) + " from " + Variant::get_type_name(p_argptrs[errorarg]->get_type()) + " to " + Variant::get_type_name(ce.expected) + ".";
+		} else {
+			err_text = "Cannot convert argument " + itos(errorarg + 1) + " from [missing argptr, type unknown] to " + Variant::get_type_name(ce.expected) + ".";
+		}
 	} else if (ce.error == Variant::CallError::CALL_ERROR_TOO_MANY_ARGUMENTS) {
 		err_text = "Method expected " + itos(ce.argument) + " arguments, but called with " + itos(p_argcount) + ".";
 	} else if (ce.error == Variant::CallError::CALL_ERROR_TOO_FEW_ARGUMENTS) {